A wind turbine installation is usually formed of a support structure comprising an elongate tower, with a nacelle and a rotor attached to the upper end of the support structure. The generator and its associated electronics are usually located in the nacelle although they may be located elsewhere, such as at the base of the support structure.
Fixed-base wind turbines that are fixed either to the land or the seabed are well known. However, recently there has been a desire to develop floating wind turbines and various structures have been proposed. One example is a wind turbine installation where a conventional wind turbine structure is mounted on a buoyant base such as a platform or raft-like structure. Another proposal is a “spar buoy” type structure. Such a structure is formed of an elongate buoyant support structure with a rotor mounted on the top. The support structure could be a unitary structure or it could be an elongate sub-structure (similar to a conventional spar buoy) with a standard tower mounted thereon.
Floating wind turbine installations may be tethered to the sea bed via one or more mooring lines with anchors, or attached to the sea bed with one or more articulated (hinged) legs, for example, in order to hold them at their desired installation sites.
In conventional wind turbines, the rotor speed is controlled in order to regulate the power output. The manner in which this is done depends upon whether the wind speed is above or below the so-called rated wind speed for the turbine. For a given wind turbine and wind speed, the aerodynamic power depends upon the power coefficient CP of the turbine. This is a function of blade pitch angle β and tip speed ratio λ. The latter is defined as the speed at which the outer tips of the rotor blades are moving divided by the wind speed. Every turbine has a characteristic optimum tip speed ratio (where CP is maximised), which is usually between 8 and 10.
The rated wind speed of a turbine is the lowest wind speed at which maximum power can be generated. When operating in winds below the rated wind speed, the control objective is to maximise power output and so the power coefficient must be maximised. This corresponds to optimum value of tip speed ratio. This operating regime is known as the maximum power regime.
The tip speed ratio may be optimised by adjusting the blade pitch angle to vary the aerodynamic torque produced by the turbine, or by adjusting the torque of the generator load experienced by the rotor. This latter arrangement is preferable because it enables the blade pitch to be set at the minimum (β=0) pitch angle (i.e. the most coarse angle), which maximises the power coefficient CP. For a given blade pitch angle the torque presented to the turbine that maximises the power coefficient can be shown to be proportional to the square of the rotor angular velocity.
In contrast, when operating above the rated wind speed, the blade pitch is adjusted with the aim of producing a constant power output regardless of variation in wind speed to prevent excessively high power outputs that could damage the generator and/or its associated electronics. This constant power is referred to as the rated power of the wind turbine. Thus, as the wind speed increases, the blade pitch is increased, i.e. made more parallel to the wind direction, in order to reduce the aerodynamic torque; in order to maintain constant power. Where the torque of the generator is variable, this can be increased to allow output power to increase even when the turbine has reached its maximum design speed. In fact, it is possible and quite common to change both pitch and generator torque above the rated wind speed in order to achieve a smooth generator power production. The generator torque, TG, is then typically controlled according to TG=PGmax/ωG, where PGmax is the maximum (or rated) generator power and ωG is the generator speed.
Floating wind turbines inevitably undergo significant movements due to the action of current, wind and waves upon them. Waves in particular cause the tower to oscillate at frequencies of about 0.05 to 0.2 Hz. These are rigid body motions (surge coupled with pitch, but mostly pitch). Usually, the size of the oscillations is minimised by modifying the geometry and weight distribution of the floating wind turbine.
However, it has been recognised that energy can be extracted from the waves by a wind turbine. As set out in WO 2005/021961, the turbine can act as a damping mechanism for wave-induced motion and thus extract energy from the waves. The amount of energy extracted from the waves depends on how the blades of the wind turbine are controlled in relation to the instantaneous velocity of the wind relative to the rotor blades. In particular, it is suggested that blade pitch be controlled in response to the motion of the tower so that thrust and power coefficients increase with increasing relative wind velocity. (An increased thrust coefficient implies a greater thrust force acting on the rotor area). The application also points out that maximum energy will be extracted if the system oscillates in resonance with the waves.
It will be appreciated that extracting wave energy in this manner is only useful when operating below the rated wind speed (the maximum power regime); when the wind speed is higher than the rated wind speed maximum output power can be obtained from wind energy alone.
As noted above in the context of conventional turbine control, in this region it is desirable for generator torque rather than blade pitch to be adjusted to maintain the optimum tip speed (and hence maximise the power coefficient). Since the optimum torque value is a function of rotor velocity it can be obtained in the steady state using conventional controllers. However, in such controllers, there is a significant delay between a change in relative wind speed and the adjustment to the torque. This is inherent in the known control systems because there is a significant time constant from the change in wind speed to the corresponding change in rotor speed (which is measured). Firstly, there is a delay between the change in wind speed and the change in the aerodynamic torque that it causes, and secondly, because of the large moment of inertia of the rotor, there is a delay between the change in aerodynamic torque and the change in rotor speed that it causes.
Such time constants are not a serious drawback in the conventional control of wind turbines because significant sustained changes in wind speed usually occur over a much longer time period. However, the combined time constant is significantly larger than the period of the wave-induced oscillations and so it is impossible to use a conventional torque controller to fully maximise energy extraction from wave-induced motion. Indeed, it can be shown that the result of using such a conventional control system is the loss of almost half of the available wave-energy.